1. Technical Field
The present invention relates to a method for mounting seals used for gas sealing at a predetermined portion in a polymer electrolyte fuel cell, and relates to fuel cells having such seals.
2. Background Art
In polymer electrolyte fuel cells, a separator plate is layered on both sides of a plate-shaped membrane electrode assembly to form a unit of the layered structure, and the plural units are layered to form a fuel cell stack. The membrane electrode assembly is a layered structure, in which an electrolyte membrane is held by gas-diffusion electrode plates at a positive side and a negative side. The separator plate is made from a material having electron transmitting characteristics, and has plural grooved gas passages in which a fuel gas such as hydrogen gas, an oxidizing gas such as oxygen or air, and a coolant flow individually. The separator plate is layered on the membrane electrode assembly such that linear protrusions between the gas passages are contacted with the gas-diffusion electrode plates.
In the fuel cell, a fuel gas is provided to the gas passage of the separator plate at the negative electrode side, and an oxidizing gas is provided to the gas passage of the separator plate at the positive electrode side, whereby electricity is generated by electrochemical reaction. During the operation of the fuel cell, the gas-diffusion electrode plates transmit the electrons generated by the electrochemical reaction between the gas-diffusion electrode plates and the separator plates, and diffuse the fuel gas and the oxidizing gas. The electrode plate at the negative electrode side produces a chemical reaction for the fuel gas so as to generate protons and electrons. The electrode plate in the positive electrode side generates water from oxygen, the protons, and the electrons, and the electrolyte membrane facilitates ionic migration for the proton, whereby the electric power is provided via the positive and negative electrode plates.
In the above-described fuel cell, the fuel gas, the oxidizing gas, and the coolant must be flowed in the individual gas passages, so that the gas passages are separated from each other by a seal. The sealing portion varies according to the structure of the fuel cell stack. For example, a seal is provided around communicating openings of the gas passages penetrating the fuel cell stack, around the membrane electrode assembly, around a coolant passage provided on the outer surface of the separator plate, and around the circumference of the outer surface of the separator plate.
According to conventional sealing technology, in general, an elastic material made from an organic rubber of the fluorine type, silicone type, ethylene propylene type, or the like, is formed into a shape of a sheet or an O-ring, and is mounted to a sealing portion. The sealing member seals the sealing portion by a reaction force generated by being compressed in a stacked condition. As other sealing structures, a seal in which an inorganic material formed of carbon or ceramics is compressed, a mechanical seal using caulking, and the like have been provided.
Fuel cells are often carried or installed in automobiles for use. In these cases, the cells are stringently required to be small and thin. Since separator plates are usually made from brittle carbon, they are readily broken during assembly of a fuel cell stack. Therefore, seals made from organic rubbers are widely used, since they are flexible and have suitable reaction force, thereby preventing breakage of the separator plate in the assembly of a fuel cell stack.
In order to mount such a seal between an electrolyte membrane of a membrane electrode assembly and a separator plate, heretofore, the membrane electrode assembly was installed in a die, a vulcanized rubber as a material for the seal was charged into a cavity formed in the die, and the material for the seal was hardened to be integrally formed with the electrolyte membrane.
Electrolyte membranes are easily deformed and wrinkled due to humidity, so that sealing properties cannot be ensured, and problems in which sufficient clamping thickness for the seal cannot be obtained occur due to variation of the thickness of the membrane electrode assembly. The mounting method in which seals are integrally formed with the membrane electrode assembly has been mentioned to be effective for overcoming such problems. However, in such a method, the vulcanization for the seal material is generally performed at a high temperature and at a high pressure, so that an excess amount of heat load is exerted on the electrolyte membrane and the electrode plate. As a result, the electrolyte membrane and the electrode plate are deteriorated, and they are contaminated and damaged during the handing thereof in the vulcanization in some cases.